Moon phase indicator devices have existed for a long time and include a moon disc on which two yellow circles symbolising the moon are drawn on a dark-coloured background, while a substantially semi-circular aperture in the dial comprises a base separated into two concave parts, all of which is well known to those skilled in the art. During the rotation of this type of moon disc underneath the aperture, the various moon phases are displayed. Conventionally, the moon phase mechanism is formed of a gear train driven by an hour wheel and by the disc on which the two representations of the moon are affixed.
For example, it is known from EP Patent No 2009517 to propose a watch equipped with such a device.
One drawback of this type of watch is that the indicator mechanism is actuated once per day, which does not provide a realistic display of the moon phases. Further, a conventional mechanism is difficult to incorporate in a movement and can result in an overload of components in the movement and thereby increase the thickness of the watch case which harms the aesthetics thereof. Finally, this type of mechanism requires the moon phase indicator to be actuated at a determined time, generally 7 pm, to avoid an accumulation of functions at midnight causing too great a loss in the amplitude of the sprung balance.